In modern, high speed sheet processing machines such as printers, sorters, collators, reproduction machines, etc. a sheet misfeed or multi-fed sheets can seriously impair the operation of the machine. Numerous devices of the type disclosed in U.S. Pat. No. 3,768,803 have been employed to minimize the possibility of misfeeds and multi-feeds.
To provide a constant normal force between the sheet being fed and the feed mechanism, it is common practice to employ a sheet elevator tray assembly having sheets stacked thereon, the feed mechanism being disposed above the stack for feeding the top sheet from the stack. As sheets are fed from the stack, the elevator is continuously shifted in an upward direction to maintain the top sheet in the stack adjacent the feed mechanism. This type of feeder is ordinarily used when a varying quantity of sheets may be loaded into the sheet tray to overcome problems encountered in bottom feed devices whereby an excessive weight or constantly varying weight of sheets may be encountered which interferes with removal of the sheet from the bottom of the stack. While the bottom feed device is more convenient since the stack may be replenished without stopping machine operation and the necessity of an elevator type sheet tray is obviated, the poor paper feeding capabilities of bottom feed devices has prevented common acceptance thereof.